To maximize your chances of fighting flab, new research offers some simple advice: Wake up early and go outside.

People who loaded up on light exposure at the beginning of the day were most likely to have a lower body mass index, said a study published in the journal PLOS ONE. That relationship between morning light and BMI was independent of how many calories the study participants consumed.

It may sound crazy, but there is sound scientific evidence to back up the link. Circadian rhythm plays an important role in regulating metabolism, and studies have shown that exposure to morning light can influence body fat and the hormones that regulate appetite.

For instance, sleep-deprived subjects whose levels of the hormones leptin and ghrelin were out of whack saw those levels improve after being exposed to light for two hours after waking up. In another study, obese women who were exposed to bright light for at least 45 minutes between 6 a.m. and 9 a.m. dropped body fat after three weeks. And studies in animals have found that altering light exposure changed their metabolism.

With all this in mind, researchers at Northwestern University’s Feinberg School of Medicine in Chicago persuaded 54 volunteers to wear a wrist monitor that measured their light exposure (including its timing and intensity) as well as their sleep patterns. The volunteers were also asked to keep a detailed log of everything they ate and drank during a seven-day period.

The volunteers (whose average age was 30) tended to be night owls — on average, they went to sleep at 1:26 a.m. and woke at 8:49 a.m. Compared with Americans as a whole, they were thin, with 58 percent reporting a body mass index of 24 or lower.

When the researchers analyzed the data, they found only one variable that correlated to BMI: MLiT. That stands for “mean light timing above threshold,” and it’s a measurement that takes into account the timing, length and brightness of each volunteer’s light exposure. Translating that into practical terms, the researchers said the key was to bask in light of at least 500 lux, and that such basking was most valuable when the exposure came early in the day. For every hour that light exposure was delayed, BMI rose by 1.28 points.

The complete mathematical model took into account demographic factors such as age and gender; the amount of sleep and exercise volunteers got; and the season of the year. But of all these variables, MLiT did the best job of predicting BMI.

There’s clearly something special about morning light. They’re not sure what it is, but one possibility is the fact that morning light contains more wavelengths in the blue portion of the spectrum. They said, “Blue light has been shown to have the strongest effect on the circadian system.”

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